The present invention relates to generating a three dimensional (3D) surface model of arbitrarily shaped objects such as dental structures.
Scanners are devices for capturing and recording information from the surface of an object. The use of scanners to determine the surface contour of objects by non-contact optical methods has become increasingly important in many applications including the in vivo scanning of dental structures to create a 3D model. Typically, the 3D surface contour is formed from a cloud of points where the relative position of each point in the cloud represents an estimated position of the scanned object's surface at the given point.
One basic measurement principle behind collecting point position data for these optical methods is triangulation. In triangulation, given one or more triangles with the baseline of each triangle composed of two optical centers and the vertex of each triangle being a target object surface, the range from the target object surface to the optical centers can be determined based on the optical center separation and the angle from the optical centers to the target object surface. If one knows the coordinate position of the optical centers in a given coordinate reference frame, such as for example a Cartesian X,Y,Z reference frame, than the relative X, Y, Z coordinate position of the point on the target surface can be computed in the same reference frame.
Triangulation methods can be divided into passive triangulation and active triangulation. Passive triangulation (also known as stereo analysis) typically utilizes ambient light and the optical centers along the baseline of the triangle are cameras. In contrast, active triangulation typically uses a single camera as one optical center of the triangle along the baseline and, in place of a second camera at the other optical center, active triangulation uses a source of controlled illumination (also known as structured light).
Stereo analysis is based upon identifying surface features in one camera image frame that are also observed in one or more image frames taken at different camera view positions with respect to the target surface. The relative positions of the identified features within each image frame are dependent on the range of each of the surface features from the camera. By observing the surface from two or more camera positions the relative position of the surface features may be computed.
Stereo analysis while conceptually simple is not widely used because of the difficulty in obtaining correspondence between features observed in multiple camera images. The surface contour of objects with well-defined edges and corners, such as blocks, may be rather easy to measure using stereo analysis, but objects with smoothly varying surfaces, such as skin or tooth surfaces, with few easily identifiable points to key on, present a significant challenge for the stereo analysis approach.
To address this challenge, fixed fiducials or a formed pattern such as dots may be placed on a target object's surface in order to provide readily identifiable points for stereo analysis correspondence. WO 98/48242 entitled METHOD AND DEVICE FOR MEASURING THREE-DIMENSIONAL SHAPES by Hans Ahlen, et. al., the content of which is incorporated by reference, discloses a method for measuring the shape of an object by first applying a pattern of paint to the object's surface and then observing the object from a multitude of positions. The pattern of paint is used in conjunction with the multiple images to perform a stereo analysis to calculate the shape of the target object's surface.
Active triangulation, or structured light methods, overcomes the stereo correspondence issue by projecting known patterns of light onto an object to measure its shape. The simplest structured light pattern is simply a spot of light, typically produced by a laser. The geometry of the setup between the light projector and the position of the camera observing the spot of light reflected from the target object's surface enables the calculation of the relative range of the point on which the light spot falls by trigonometry. Other light projection patterns such as a stripe or two-dimensional patterns such as a grid of light dots can be used to decrease the required time to capture the images of the target surface.
The measurement resolution of the target objects' surface features using structured lighting methods is a direct function of the fineness of the light pattern used and the resolution of the camera used to observe the reflected light. The overall accuracy of a 3D laser triangulation scanning system is based primarily upon its ability to meet two objectives: 1) accurately measure the center of the illumination light reflected from the target surface and 2) accurately measure the position of the illumination source and the camera at each of the positions used by the scanner to acquire an image.
To achieve the second objective, commercial 3D scanners typically utilize precision linear or rotational stages to accurately reposition either the illuminator/camera pair or the target object between image acquisitions. However, a variety of real-world situations such as 3D imaging of intra oral human teeth do not lend themselves to the use of conventional linear or rotational stages. Further, the great range in sizes and shapes of the human jaw and dentition make the use of a single fixed path system impractical.
Commercially available 3D scanner systems have been developed for the dental market that accommodate the variety of human dentition by incorporating an operator held, wand type scanner. In these systems, the operator moves the scanner over the area to be scanned and collects a series of image frames. In this case however, there is no known positional correspondence between image frames because each frame is taken from an unknown coordinate position that is dependent upon the position and orientation of the wand at the instance the frame was taken. These handheld systems must therefore rely on scene registration or the application of an accurate set of fiducials over the area to be scanned. For example, U.S. Pat. No. 6,648,640 entitled INTERACTIVE ORTHODONTIC CARE SYSTEM BASED ON INTRA-ORAL SCANNING OF TEETH by Rudger Rubbert et. al., the content of which is incorporated by reference, discloses a scanner which acquires images of the denture which are converted to three-dimensional frames of data. Pattern recognition can then be used to register the data from several frames to each other to provide a three-dimensional model of the teeth.
For 3D structures such as teeth, the use of pattern recognition or fidicials for frame registration is not optimal since tooth surfaces do not always provide sufficient registration features to allow high accuracy scene registration and accurate placement of fiducials to the required resolution is impractical over anything but the smallest tooth. U.S. Pat. No. 4,837,732 entitled METHOD AND APPARATUS FOR THE THREE-DIMENSIONAL REGISTRATION AND DISPLAY OF PREPARED TEETH and U.S. Pat. No. 4,575,805 entitled METHOD AND APPARATUS FOR THE FABRICATION OF CUSTOM-SHAPED IMPLANTS, both by Brandestini and Moermann, and whose contents are incorporated by reference, disclose a scanning system for in vivo, non-contact scanning of teeth and a method for optically mapping a prepared tooth with a non-contact scan-head. The non-contact scanner includes a light emitting diode which is used in conjunction with a plurality of slits to form a structured light pattern on a tooth's surface. The reflected light is recorded by a linear charge coupled device sensor array. Triangulation is used to map the surface contour of the scanned teeth.
U.S. Pat. No. 5,372,502 entitled OPTICAL PROBE AND METHOD FOR THE THREE-DIMENSIONAL SURVEYING OF TEETH by Massen et al., the content of which is incorporated by reference, discloses an optical based scanner for measuring the surface contour of teeth that has a similar principle of operation. As noted in the Massen et al. patent, the Biandestini et al. technique is difficult to use when there are large variations in surface topography since such large variations in the surface displace the pattern by an amount larger than the phase constant of the pattern, making it difficult to reconstruct the pattern of lines. Furthermore, precise knowledge of the angle of incidence and angle of reflection, and the separation distance between the light source and the detector, are needed to make accurate determinations of depth. Furthermore, the scanner has to be rather carefully positioned with respect to the tooth and would be unable to make a complete model of a jaw's dental structure.
U.S. Pat. No. 5,027,281 entitled METHOD AND APPARATUS FOR SCANNING AND RECORDING OF COORDINATES DESCRIBING THREE DIMENSIONAL OBJECTS OF COMPLEX AND UNIQUE GEOMETRY by Rekow et. al., the content of which is incorporated by reference, discloses a scanning method using a three axis positioning head with a laser source and detector, a rotational stage and a computer controller. The computer controller positions both the rotational stage and the positioning head. An object is placed on the rotational stage and the laser beam reflects from it. The reflected laser beam is used to measure the distance between the object and the laser source. X and Y coordinates are obtained by movement of the rotational stage or the positioning head. A three-dimensional virtual model of the object is created from the laser scanning. Thus, a plaster model of teeth can be placed on a rotational stage for purposes of acquiring shape of the teeth to form a pattern for a dental prosthesis.
U.S. Pat. No. 5,431,562 entitled METHOD AND APPARATUS FOR DESIGNING AND FORMING A CUSTOM ORTHODONTIC APPLIANCE AND FOR THE STRAIGHTENING OF TEETH THEREWITH by Andreiko et al., the content of which is incorporated by reference, describes a method of acquiring certain shape information of teeth from a plaster model of the teeth. The plaster model is placed on a table and a picture is taken of the model's teeth using a video camera positioned a known distance away from the model, looking directly down on the model. The image is displayed on an input computer and a positioning grid is placed over the image of the model teeth. The operator manually inputs X and Y coordinate information of selected points on the model teeth, such as the mesial and distal contact points of the teeth. An alternative embodiment is described in which a laser directs a laser beam onto a model of the teeth and the reflected beam is detected by a sensor. Neither technique achieves in vivo scanning of teeth.
Systems and methods have been developed that allow in vivo scanning of teeth while avoiding the need to perform pattern recognition or use fiducials for image frame registration. In these systems the accurate surface contour of a scanned object is computed from a series of active triangulation image capture frames where each frame is obtained from precisely known positions of the image aperture. U.S. Pat. No. 6,592,371 entitled METHOD AND SYSTEM FOR IMAGING AND MODELING A THREE DIMENSIONAL STRUCTURE by Durbin, et. al., the content of which is incorporated by reference, discloses a method for optically imaging the dental structure using one or more image apertures movably coupled to an intra-oral track in a manner that results in each captured image frame being obtained from a known position with respect to all other captured images. By gathering each image frame through an image aperture that is at a known position and orientation as the aperture traverses along an intra oral track this method allows the 3D surface contour of the teeth and jaw dentia to be directly computed without performing frame registration.
The intra oral cavity represents a significant challenge for accurate in vivo 3D imaging of the surface of teeth and tissue. The ability to accurately measure the center of a scanning line is affected by the translucency of teeth, the variety of other reflecting surfaces (amalgam fillings, metal crowns, gum tissue, etc.) and the obscuration due to adjacent surfaces. Further, linear or rotational motion adds to error accumulation and the variation in size and curvature of human jaws makes a “one size fits all” scanner problematic.